1. Field of the Invention
The present invention relates to an electro-photographic photoreceptor excellent for repeated use.
2. Description of the Related Art
Known electrophotographic photoreceptors having a photosensitive layer composed of inorganic photoconductive compounds, such as selenium, zinc oxide and cadmium sulfide, have been made.
However, these are not satisfactory in sensitivity, heat stability, moisture resistance and endurance. Selenium and cadmium sulfide, in particular, have a limited use in production and handling because of their toxicity.
Known electrophotographic photoreceptors having a photosensitive layer composed of organic photoconductive compounds have been made. These photoreceptors have many merits in that they are relatively easy to produce and handle, are inexpensive and generally superior to selenium photoreceptors in heat stability.
Poly-N-vinylcarbazole is a well known organic photoconductive compound. An electrophotographic photoreceptor having a photosensitive layer composed of a charge transfer complex formed from poly-N-vinylcarbazole and a Lewis acid such as 2,4,7-trinitro-9-fluorenone is disclosed in Japanese Patent Kokoku No. 50-10496. However, this photoreceptor is not satisfactory in sensitivity, film formability and endurance.
Organic photoconductors of low molecular weight represented by hydrazone and pyrazolines have been tried. Film-formability is considerably improved by combining these organic photoconductors with suitable binders. However, the improvement in sensitivity and endurance is insufficient to warrant their use.
Recently, a double-layered type photoreceptor in which a carrier generating function and a carrier transporting function are born on separate substances, has been proposed. Employing this structure has resulted in remarkable improvements in charging characteristics and sensitivity. Photoreceptors having a sensitivity near that of inorganic photoreceptors, such as Se, can be made by combining a carrier generation layer comprising an azo pigment, which has a high carrier generating ability, with a carrier transport layer containing a hydrazone type carrier transport material, which has a high carrier transporting ability. As a result, photoreceptors composed of organic photoconductive compounds of these types are now being used in copy machines and printers. However, electro-photographic photoreceptors made of these organic materials are not stable when used repeatedly in a copy machine. After repeated use, the initial potential decreases and the residual potential after removal of charge increases.